First Law of Thermodynamics

Definition

The First Law of Thermodynamics, also known as the Law of Conservation of Energy, states that energy can neither be created nor destroyed; it can only be transformed from one form to another or transferred between a system and its surroundings. In any thermodynamic process, the total energy of an isolated system remains constant.

Main Content

1. The Concept of Internal Energy

• Internal energy (U) is the total microscopic kinetic and potential energy associated with the atoms and molecules of a system.
• When heat is added to a system or work is done on it, the internal energy of the system increases, causing a rise in temperature or a change in state.

2. The Energy Balance Equation

• The law is mathematically expressed as: ΔU = Q - W
• Where ΔU is the change in internal energy, Q is the net heat added to the system, and W is the work done by the system.
• If heat enters the system, Q is positive. If the system does work on its surroundings, W is positive.

3. Energy Transfer Mechanisms

• Energy crosses the boundaries of a system primarily in two ways: Heat (energy transfer due to temperature difference) and Work (energy transfer due to a force acting through a distance).
• The law implies that for a cycle (where the system returns to its initial state), the total heat transfer must equal the total work transfer.

       Surroundings
    +-----------------+
    |      Heat (Q)   |
    |      ----->     |
    |  +-----------+  |
    |  |  System   |  |
    |  +-----------+  |
    |      ----->     |
    |      Work (W)   |
    +-----------------+

Working / Process

1. Identifying the System Boundaries

• Define the specific region of space or the quantity of matter under study. Everything outside this boundary is considered the "surroundings."
• Determine if the system is closed (mass cannot cross the boundary) or open (mass and energy can cross).

2. Measuring Energy Transfers

• Measure the amount of heat (Q) entering or leaving the system, usually through calorimetry or temperature gradient analysis.
• Calculate the work (W) done by or on the system, often by measuring pressure changes and volume displacement (W = ∫P dV).

3. Calculating the Net Change

• Apply the First Law formula: ΔU = Q - W.
• If the calculated ΔU is positive, the system has stored energy. If negative, the system has lost energy to the surroundings.

Advantages / Applications

• Engine Design: Used by engineers to calculate the efficiency of internal combustion engines and power plants by tracking energy flow.
• Refrigerator Performance: Helps in understanding how electrical work is used to extract heat from a cold space to a warmer environment.
• Biological Systems: Explains how the human body converts the chemical energy stored in food into mechanical work and heat to maintain body temperature.

Summary

The First Law of Thermodynamics is the foundational principle that energy is always conserved, acting as a balance sheet for energy inputs and outputs. It dictates that for any physical process, the change in a system's internal energy is equal to the heat supplied minus the work performed.

• Conservation of Energy: Energy is never created or destroyed, only converted.
• Energy Balance: ΔU = Q - W is the core mathematical relationship.
• Universal Application: Applicable to everything from steam engines to complex biological organisms.
• Important terms: Internal Energy (U), Heat (Q), Work (W), and Isolated System.